A gear transmission mechanism includes a housing, a first transmission assembly, a second transmission assembly, and a gear clearance adjustment assembly. The first transmission assembly includes an input shaft, and the second transmission assembly includes an output shaft. The gear clearance adjustment assembly includes an adjustment base mounted on the housing, fasteners, bearings, a washer, a fixing member, and a pair of freely rotatable adjusting gears for the transmission of rotative force. The adjusting gears mesh with the first and second transmission gears.